The present invention relates to a thermoplastic elastomer, and is particularly concerned with an energetic thermoplastic elastomer having urethane moieties as its thermoplastic A segments.
Thermoplastic elastomers typically consist of copolymer chains having monomers A and B distributed throughout the chains as ABA or AB, where the A is the hard segment providing the thermoplastic characteristic and B is the soft segment providing the elastomeric behavior to the polymer. Conventionally , the A segment is formed by a crystalline homopolymer and the soft segment is formed by an amorphous homopolymer.
Thermoplastic elastomers of the type ABA are usually obtained by polymerization the soft B segment followed by the addition of the hard A segment, which is crystallisable. To achieve this type of copolymerization, monomers of both types should have similar reactivity to provide a copolymer of controlled structure with suitable adjustable mechanical properties. A good example of this type of technology is the preparation of 3-azidomethyl-3-methyloxetane and 3.3-bis(azidomethyl)oxetane (AMMO/BAMO) energetic thermoplastic elastomer described in U.S. Pat. No. 4,707,540 to Manser et al. and U.S. Pat. No. 4,952,644 to Wardle et al. In this energetic thermoplastic elastomer (ETPE), the thermoplastic part is obtained by the crystallization of the BAMO polymer. Manser et al. also described the use of these AMMO/BAMO energetic homopolymers as prepolymers in making thermoset binders for use in propellants. To obtain the thermoset binders, Manser et al. would typically cure the AMMO/BAMO prepolymers with a triol and diisocyanate to form a chemically cross-linked matrix to obtain the desired binder.
In the case of copolymers of the type AB, the thermoplastic elastomers are usually obtained by mixing monomers that have compatible reactive ending groups. U.S. Pat. No. 4,806,613 to Wardle describes such a method of synthesis. Similarly to Manser et al., Wardle also uses BAMO as the crystalline hard segment. For this, he end capped both the A and B homopolymers with toluene diisocyanate (TDI) leaving at each end an unreacted isocyanate, mixing both homopolymers and joined them by using a small chain extender. Alternatively, Wardle used a block linking technique consisting of reacting the B block with phosgene or a diisocyanate followed by the addition of the A block to form the thermoplastic elastomer. Once again, the crystalline homopolymer BAMO which is an expensive starting material is required to form the hard segment of the thermoplastic elastomer.
It is an object of the present invention to provide an energetic thermoplastic elastomer that is inexpensive to produce by avoiding the use a crystalline homopolymer to form the A segment.
In accordance with one aspect of the present invention, there is provided a thermoplastic elastomer comprising copolymer chains having urethane moieties physically bonded to one another by hydrogen bonds to yield the hard segment of the thermoplastic elastomer.
More specifically, the thermoplastic elastomer of the present invention have copolymer chains, which may be represented by the formulae:
HOxe2x80x94Pxe2x80x94(Uxe2x80x94P)nxe2x80x94OHxe2x80x83xe2x80x83(I)
wherein P is selected from the group consisting of 
where the
R1 groups are the same and selected from the group consisting of xe2x80x94CH2N3 and xe2x80x94CH2ONO2;
R2 is selected from the group consisting of xe2x80x94OCH2CH2Oxe2x80x94, xe2x80x94OCH2CH2CH2Oxe2x80x94 and xe2x80x94OCH2CH2CH2CH2Oxe2x80x94; and o and p are each xe2x89xa71; and 
xe2x80x83where the
R3 groups are the same and selected from the group consisting of xe2x80x94CH2N3 or xe2x80x94CH2ONO2 when R4 are xe2x80x94CH3; or R3 and R4 are both xe2x80x94CH2N3 
R5 is selected from the group consisting of xe2x80x94OCH2CH2Oxe2x80x94, xe2x80x94OCH2CH2CH2Oxe2x80x94 and xe2x80x94OCH2CH2CH2CH2Oxe2x80x94; and q and r are both xe2x89xa71
U is selected from the group consisting of 
xe2x80x83and n is 1 to 100;
wherein the A block is provided by said U moieties and the B block is provided by the P moieties.
Preferably, P has a molecular weight ranging from about 500 to about 10,000.
In accordance with another aspect of the present invention, the thermoplastic elastomer further comprises a chain extender such as 
or xe2x80x94OCH2xe2x80x94(CH2)nxe2x80x94CH2Oxe2x80x94 where n is 0 to 8.
In the presence of a chain extender, the copolymer chains of the thermoplastic elastomer of the present invention may further be described with the following structure:
HOxe2x80x94Pxe2x80x94(Uxe2x80x94(Cxe2x80x94U)axe2x80x94P)bxe2x80x94Uxe2x80x94Pxe2x80x94OHxe2x80x83xe2x80x83(II)
wherein P, U and C, which is the chain extender, are defined above; a is 1 to 100 and b is 1 to 100.
Alternatively, the copolymer chains may have the following structure:
HOxe2x80x94Pxe2x80x94Uxe2x80x94(Cxe2x80x94U)xxe2x80x94(Pxe2x80x94U)yxe2x80x94(Cxe2x80x94U)zxe2x80x94Pxe2x80x94OHxe2x80x83xe2x80x83(III)
wherein P, U and C are defined as above; and x, y and z are each 1 to 100.
The thermoplastic elastomer of the present invention is produced by drying a dihydroxyl terminated telechelic energetic prepopolymer having a functionality of two or less, and polymerizing the dried energetic prepolymer with a diisocyanate at a NCO/OH ratio ranging from about 0.7 to about 1.2, and preferably about one, under dried conditions. The use of dried reactants couple with providing a dried environment, i.e. avoiding the presence of water, during the polymerization step prevent the formation of undesired covalent bonds between the growing chains (chemical crosslinkings). This may be further prevented by purifying the diisocyanate prior to its use.
Preferably, the reaction is performed in the presence of a suitable catalyst such as dibutyltin dilaurate, which is added to the prepolymer prior to drying the latter to ensure its perfect dispersion in the prepolymer.
Suitable prepolymers are glycidyl azide polymer, 3-azidomethyl-3-methyloxethane, 3-nitratomethyl-3-methyloxetane, 3,3-bis(azidomethyl)oxetane and glycidyl nitrate that have molecular weights ranging from about 500 to about 10,000.
Suitable diisocyanates are 4,4xe2x80x2 methylenebis-phenyl isocyanate, toluene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate.
Chain extenders such as 2,4-pentanediol, 1,3-propanediol, 1,4-butanediol or a diol having the formula: HOxe2x80x94CH2xe2x80x94(CH)nxe2x80x94CH2xe2x80x94OH where n is 0 to 8 may be added to vary the thermoplastic content of the copolymer and the mechanical properties of the thermoplastic elastomer.
The present invention provides an energetic thermoplastic elastomer (ETPE) having linear copolymer chains having the formulae:
HOxe2x80x94Pxe2x80x94(Uxe2x80x94P)nxe2x80x94OHxe2x80x83xe2x80x83(I)
wherein the macromonomers P are derived from energetic dihydroxyl terminated telechelic polymers having a functionality of two or less such as poly glycidyl azide polymer (GAP), poly 3-azidomethyl-3-methyloxetane (AMMO), poly bis 3,3-azidomethyloxetane (BAMO), poly 3-nitratomethyl-3-methyloxetane (NIMMO) and poly glycidyl nitrate (GLYN), with poly GAP being the most preferred compound.
U are components of diisocyanates such as 4,4xe2x80x2 methylenebis-phenyl isocyanate (MDI), toluene diisocyanate (TDI), hexamethylene diisocyanate (HMDI) and isophorone diisocyanate (IPDI).
The energetic thermoplastic elastomer of the present invention may further include a chain extender. Suitable chain extenders are: 
and xe2x80x94OCH2xe2x80x94(CH2)nxe2x80x94CH2Oxe2x80x94 where n is 0 to 8.
In the present invention, the chain extenders serve a dual purpose. As usual, these chain extenders can be used to increase the molecular weight of the copolymers, but unlike conventional chain extenders, they are also used to increase the hard segment in the energetic thermoplastic elastomer.
The energetic copolymer (I) of the present invention is obtained by polymerizing a dihydroxyl terminated telechelic energetic polymer having a functionality of two or less such as poly glycidyl azide polymer, poly 3-azidomethyl-3-methyloxethane, poly 3-nitratomethyl-3-methyloxetane and poly glycidyl nitrate with a diisocyanate such as 4,4xe2x80x2 methylenebis-phenyl isocyanate, toluene diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate at a NCO/OH ratio ranging from about 0.7 to about 1.2 under dried conditions. The most preferred ratio is about one. The resulting copolymers comprise urethane groups which form hydrogen bonds between the chain of copolymers to yield the hard segment in the copolyurethane thermoplastic elastomer. In contrast to the prior art processes, the process of the present invention is cheap in that an expensive crystalline homopolymer, for example BAMO, is not required.
In a more specific example, the following structure (IV) is obtained by the polymerization of GAP with 4,4xe2x80x2 methylenebis-phenyl isocyanate. 
In this copolymer, the elastomeric B segment is provided by the amorphous GAP component and the thermoplastic A segment is provided by the urethane moieties of the MDI component. Each urethane group within the copolymer is capable of forming hydrogen bonds with the oxygen of another urethane or with the oxygen of an ether. By doing so, physical cross-links are obtained between the chains. These physical cross-links are reversible and hence, can be broken by melting or dissolving the copolymer in a suitable solvent so that the polymer can be mixed with other components in, for example, a gun propellant formulation. Such a gun propellant can be isolated upon cooling or evaporating the solvent. Cooling or evaporating the solvent lets the broken physical cross-links, i.e. hydrogen bonds, reform to recover the thermoplastic elastomer, thus providing a recyclable product.
In most case, it is also possible to break the hydrogen bonds by melting them. However, in the case of GAP-based copolyurethane thermoplastic elastomers, the copolyurethane should not be melted as both the decomposition of GAP and the melting point of the polyurethanes occur at about 200xc2x0 C. Generally, linear polyurethanes have melting points in the region of 200xc2x0 C. when the thermoplastic content is about 20 to 50% by weight. This is when there is enough hard segments to induce crystallinity.
To obtain the best reproducible thermoplastic elastomer, precautions should be applied to avoid cross-linkings or the formation of covalent bonds. The dihydroxyl terminated telechelic energetic prepolymer should have a functionality of two or less. Branched prepolymers or tri or tetra-functional prepolymers would lead to the formation of undesired chemical bonds (crosslinking) which will lead to a thermoset elastomer instead of a thermoplastic elastomer. In reacting the prepolymer with the diisocyanate, the concentration of isocyanate and hydroxyl groups, i.e. NCO/OH ratio, should preferably be kept between about 0.7 to about 1.2, and most preferably about one to yield linear copolyurethane chains. An excess of isocyanate will yield allophanate or biuret group formation, leading to undesirable covalent cross-linkings.
The reaction should also be performed under dried conditions, i.e. avoiding the presence of water. This generally includes drying the dihydroxyl terminated energetic prepolymers before their polymerization and performing the polymerization step under dried conditions.
If present, water will compete with the hydroxyl group of the prepolymers and react with the isocyanate to yield a carbamic acid which decomposes to liberate carbon dioxide and form an amine group. This amine group reacts with isocyanate, yielding an urea group which introduces rigidity and brittleness to the polyurethane. Moreover, this urea group can react with another isocyanate to give a biuret group, thus introducing covalent cross-linking between the copolymer chains. This is mostly important when using prepolymers having secondary hydroxyl group such as GAP and GLYN since water has a reactivity towards isocyanates similar to that of a secondary hydroxyl group. Whereas, the reactivity of primary hydroxyl groups toward isocyanates is ten times the reactivity of water and thus it is less important to avoid the presence of water when using prepolymers containing primary hydroxyl groups such as AMMO, BAMO and NIMMO.
The formation of hydrogen bonds are optimal with linear copolyurethanes when the molecular weight is the highest and this is obtained when using a NCO/OH ratio of about one. Such copolyurethanes will provide a good alignment between the copolymers chains which promotes the formation of a high number of hydrogen bonds especially when the diisocyanates are aromatic since the aromatic rings have a great tendency to stacking-up resulting in a perfect alignment of the urethane moieties. The mechanical properties of the copolymers are directly related to the number of hydrogen bonds formed. A high degree of alignment will result in the formation of a high number of hydrogen bonds. This gives a strong hard segment domains and therefore, a strong copolyurethane thermoplastic elastomers. Hence, better quality energetic thermoplastic elastomers are obtained as the NCO/OH approaches one.
A suitable catalyst such as dibutyltin dilaurate can be used to ensure a complete reaction. Preferably, the catalyst is mixed with the prepolymers before the latter is dried to ensure that it is well dispersed in the prepolymer.
Preferably, the diisocyanate is purified prior to its use. This applies mostly to MDI which has a high reactivity towards water and in its presence will form an amino isocyanate or a diamino compound. This compound will introduce chemical cross-linking.
Chain extenders such as ethylene glycol, 1,3-propanediol, 1,4-butanediol and 2,4-pentanediol or other low molecular weight diols may be added to increase the content of hard segments in the thermoplastic elastomer. The addition of chain extenders results in the formation of localized diurethane groups within the copolymer and consequently more hydrogen bonding leading to stronger hard segment domains and stronger copolyurethane thermoplastic elastomers. When using chain extenders, one should increase the amount of diisocyanates in order to keep the NCO/OH at the desired ratio.
The chain extenders can be mixed with the diisocyanate before the addition of the prepolymers or they could be mixed simultaneously with the prepolymers and diisocyanate. In the former case, one would obtain segments consisting of consecutive xe2x80x94Uxe2x80x94Cxe2x80x94 units leading to a linear copolyurethane having the following general formula:
HOxe2x80x94(Pxe2x80x94(Uxe2x80x94(Cxe2x80x94U)axe2x80x94P)bxe2x80x94Uxe2x80x94Pxe2x80x94OH
wherein
P is selected from the group consisting of 
xe2x80x83where the
R1 groups are the same and selected from the group consisting of xe2x80x94CH2N3 and xe2x80x94CH2ONO2;
R2 is selected from the group consisting of xe2x80x94OCH2CH2Oxe2x80x94, xe2x80x94OCH2CH2CH2Oxe2x80x94 and xe2x80x94OCH2CH2CH2CH2Oxe2x80x94; and o and p are each xe2x89xa71; and 
xe2x80x83where the
R3 groups are the same and selected from the group consisting of xe2x80x94CH2N3 or xe2x80x94CH2ONO2 when the R4 groups are xe2x80x94CH3; or R3 and R4 are both xe2x80x94CH2N3 
R5 is selected from the group consisting of xe2x80x94OCH2CH2Oxe2x80x94, xe2x80x94OCH2CH2CH2Oxe2x80x94 and xe2x80x94OCH2CH2CH2CH2Oxe2x80x94; and q and r are both xe2x89xa71
U is selected from the group consisting of 
C is selected from the group consisting of 
xe2x80x83and
xe2x80x94OCH2xe2x80x94(CH2)nxe2x80x94CH2Oxe2x80x94 where n is 0 to 8.
a is 1 to 100 and b is 1 to 100
This results in very localized hydrogen bonds leading to a hard rubber.
In the latter case, the xe2x80x94Cxe2x80x94Uxe2x80x94 unit is more distributed statistically within the copolymer yielding a copolyurethane having a linear copolyurethane chain having the following formulae:
HOxe2x80x94Pxe2x80x94Uxe2x80x94(Cxe2x80x94U)xxe2x80x94(Pxe2x80x94U)yxe2x80x94(Cxe2x80x94U)zxe2x80x94Pxe2x80x94OH
wherein P, U and C are defined as above, and x, y and z are each an integer from 1 to 100.
This will result in a softer rubber than the former case.
Preferably, chain extenders having primary hydroxyl groups are used with energetic prepolymers having primary hydroxyl groups. For example, ethylene glycol would be a good candidate for the polymerization of ETPE based on AMMO or NIMMO prepolymers. Likewise chain extenders having secondary hydroxyl groups such as 2,4-pentanediol is better suited for energetic prepolymers having secondary hydroxyl groups such as GAP and GLYN since the reactivity of the hydroxyl groups are similar.
The properties of the energetic thermoplastic elastomer can also be modified by varying the type of prepolymers used. For example, in applying the process of the present invention to amorphous prepolymers such as GAP, GLYN, AMMO and NIMMO, the resulting product is a rubber having elastomeric properties. However, if the process is applied to a thermoplastic prepolymer such as BAMO, the final product will a hard wax.
The polymerization step in accordance with the method of the present invention may also be performed in a suitable solvent such as ethyl acetate to avoid the solvation step which is necessary if the copolymer is to be used as a component of, for example, a gun propellant obtained using a solvent process.
The present invention is further described in the following non-limiting examples.
GAP Mn=2000 was obtained from 3M company, Minnesota, U.S.A. Dibutyltin dilaurate and 4,4xe2x80x2 methylenebis-phenyl isocyanate were obtained from Aldrich Chemical Co., Milwaukee, Wis., U.S.A. Poly-NIMMO Mn=2000 was obtained from ICI England.